1. Field of the Invention
This invention relates to a radiographic image reading method that is applicable to a radiographic imaging system. More particularly, this invention relates to an optical means of eliminating impediment rays, which is used for reading a radiographic image in the radiographic imaging system in which a radiographic image is recorded on a stimulative phosphor material and is then reproduced by irradiating it with stimulative excitation rays.
2. Description of the Prior Art
A radiographic image such as an X-ray image is popularly used for diagnosing diseases. To obtain such X-ray images, there utilize the so-called radiographs which are produced in the manner that a phosphor layer such as an intensifying screen is irradiated with X-rays which have penetrated a subject so as to produce visible rays, and a silver salt-containing film is exposed to the visible rays and is then developed in a process similar to those taken in the ordinary photography.
In recent years, however, there have been devised the methods of obtaining images directly from a phosphor layer without using any films coated thereon with a silver salt.
These methods include a method in which radiation rays which have penetrated a subject are absorbed in a phosphor material so as to store radiant energy therein, and then the radiant energy is radiated in the form of fluorescence by exciting it with the energy of light or heat for example, and an image is formed by detecting the radiated fluorescence. U.S. Pat. No. 3,859,527 and Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 12144/1980, for example, disclose a radiographic image conversion method in which a stimulative phosphor material is used, and visible or infrared rays of light are served as stimulative excitation rays. This method uses a radiographic image conversion panel comprising a support bearing thereon a stimulative phosphor material layer. In this method, an image is obtained in such a manner that the stimulative phosphor material layer of this radiographic image conversion panel is irradiated with radiation rays which have penetrated a subject and it stores therein the radiant energy corresponding to the penetration degree of the radiation rays so penetrated each region of the subject as to form a latent image, and then each of the regions of the stimulative phosphor material layer is applied with the stored radiant energy latent-imagewise by scanning over the layer with stimulative excitation rays and the radiant energy is converted into rays of light, and the image is formed according to the optical signals generated by the various intensity of the converted rays of light. From this finished image, a hard-copy or an image on a CRT may further be reproduced.
In order to obtain a noiseless and sharp radiographic image in such a radiographic imaging system, it is required to efficiently separate the above-mentioned stimulative excitation rays and stimulative luminescence from each other, and it is, in addition, required to separate them from each other without losing the stimulative luminescence as far as possible, because an S/N ratio is extremely lowered when the stimulative excitation rays strike on a photodetector.
The above-mentioned methods include, for example, such a method of separating stimulative excitation rays and a stimulative luminescence from each other by means of a filter, as proposed in Japanese Patent O.P.I. Publication Nos. 12142/1980, 12144/1980, 12145/1980 and the like.
Every effort to improve the above-mentioned S/N ratio was not yet rewarded with satisfactory result.